The present invention relates generally to semiconductor devices and, more particularly, to controlling ferroelectricity in dielectric films by process-induced strain therein.
Integrated ferroelectrics have many current or potential future uses in microelectronics, including, for example, ferroelectric field effect transistor (FET) memory, ferroelectric metal-insulator-metal (MIM) capacitor memory, and ultra low-power/voltage complementary metal oxide semiconductor (CMOS) logic, to name a few.
At present, there are few good candidate ferroelectric materials for such applications, due to a large number of requirements (e.g., a ferroelectric transition temperature (Tc) well above room temperature, high remanent polarization, good retention, low fatigue, etc.). One such production-worthy material is lead zirconate titanate (Pb[ZrxTi1-x]O3 0<x<1, or PZT by its chemical formula). PZT is a ceramic material with a perovskite crystal structure that shows substantial ferroelectricity, i.e., generation of spontaneous electric polarization (electric dipoles) in the presence of an electric field. However, one disadvantage of using PZT in microelectronic applications is the introduction of lead (Pb) into the manufacturing line, which creates environmental concerns. Also, PZT exhibits a considerable loss of switchable polarization with cumulative switching cycles.
Another such ferroelectric material is SrBi2Ta2O9 or SBT. One disadvantage associated with SBT (in addition to the complexity of the composition of SBT, having three metal ions) relates to process control concerns, such as high processing temperatures. Other potential ferroelectric candidates have too low a transition temperature, Tc, or too low a spontaneous or remanent polarization, Pr, for some applications. For example, BaTiO3 has a Tc of about 120° C., which is too close to room temperature for applications at room temperature.
Consequently, other approaches have focused on significantly enhancing and/or tuning Tc or Pr in ferroelectric materials through the introduction of biaxial strain. Biaxial strain in ferroelectric thin films has thus far been achieved experimentally by coherent epitaxy of the ferroelectric material on a substrate (e.g., oxide) with low lattice mismatch. For example, biaxial strain in BaTiO3 thin films (achieved by coherent epitaxy on scandate substrates such as DyScO3 or GdScO3) can result in a ferroelectric transition temperature Tc nearly 500° C. higher and a remanent polarization Pr at least 250% higher than in bulk BaTiO3 single crystals. In this case, the strain is biaxial and compressive. In addition, biaxial strain may also be achieved in other ferroelectrics such as PbTiO3 or BiFeO3 via epitaxy. Yet other approaches have focused on inducing ferroelectricity in normally non-ferroelectric materials through the introduction of biaxial strain. For example, biaxial strain in SrTiO3 films (achieved by coherent epitaxy on scandate substrates such as DyScO3 or GdScO3) can result in ferroelectricity at room temperature.
However, the biaxial strain of ferroelectrics via epitaxy has its own limitations. For example, direct epitaxy on silicon requires molecular beam epitaxy (MBE) deposition for high quality epitaxy. Here, the strain cannot be tuned, and once a critical thickness is achieved the strain is relaxed. Thus, the modulated ferroelectric properties can be obtained only for a limited thickness range. Moreover, direct epitaxy of a ferroelectric oxide such as BaTiO3 on Si leads to high leakage currents due to a negative or very small band offset with respect to the silicon conduction band.